The present invention relates to a pneumatic tire having a tread which is composed of a unique blend of latices. The tread composition exhibits improved properties which correlate to increased durability and treadwear of the pneumatic tire.
The present invention relates to a pneumatic tire having a tread containing from 50 to 100 phr of a dried rubber derived from a blend of a styrene butadiene latex and an acrylonitrile butadiene latex.
There is disclosed a pneumatic tire having a tread comprised of 100 parts by weight of a first blend of rubbers, wherein said first blend comprises
(a) 50 to 100 phr of a dried rubber derived from a second blend of rubber latices, said second blend of rubber latices containing styrene-butadiene rubber latex and acrylonitrile-butadiene rubber latex and
(b) 0 to 50 phr of a dried rubber selected from the group consisting of natural rubber, synthetic cis 1,4-polyisoprene, 3,4-polyisoprene, styrene/butadiene rubber, acrylonitrile/butadiene rubber, styrene/isoprene/butadiene terpolymer rubber, cis 1,4-polybutadiene rubber, high trans 1,4-polybutadiene rubber, styrene/isoprene rubber isoprene/butadiene rubber and mixtures thereof.
A majority (from 50 to 100 phr) of the total rubber used in the tread is a dried rubber derived from a blend of two rubber latices. The first latex is a latex derived from the emulsion polymerization of styrene and butadiene monomers. The bound styrene content of the styrene butadiene rubber latex may range from 5 to 50 percent by weight. For example, conventional styrene contents in commercially available SBR generally range from 20 to 28 percent and 30 to 45 percent. The relatively higher levels of styrene are considered beneficial for enhancing traction or skid resistance of the tire tread. The solids content, also known as percent solids, of the styrene butadiene latex may vary. For example, the percent solids may range from 5 to 70 percent. Preferably, the percent solids range from 10 to 40 percent.
The other latex which is combined with the styrene butadiene latex is an acrylonitrile butadiene latex, also known as an NBR latex. The bound acrylonitrile content may range as low as 5 percent to as high as 50 percent by weight. Preferably, the acrylonitrile level ranges from 10 to 35 percent. The solids content may range from 5 to 70 percent. Preferably, the percent solids range from 10 to 40 percent.
The SBR latex and the NBR latex are prepared in accordance with conventional emulsion polymerization techniques. The principles of emulsion polymerization are discussed in references such as xe2x80x9cSynthetic Rubberxe2x80x9d by G. S. Whitby, Editor-in-Chief, John Wiley and Sons, 1954, particularly Chapter 8, and xe2x80x9cEmulsion Polymerizationxe2x80x9d by F. A. Bovey et al, Vol. IX of xe2x80x9cHigh Polymers,xe2x80x9d Interscience Publishers, Inc., 1955.
Conventional free radical polymerization initiators that are used in emulsion polymerization include compounds such as t-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide and paramenthane hydroperoxides and even hydrogen peroxide. These compounds perform very effectively when used in polymerization recipes containing appropriate levels of supporting ingredients. By xe2x80x9csupporting ingredientsxe2x80x9d is meant those materials often referred to as activators in emulsion, or other systems where required. U.S. Pat. No. 3,080,334 describes some of these materials at column 5, lines 20-26. Such materials can also be referred to as catalyst activators. The term xe2x80x9credox polymerizationxe2x80x9d is often used where the complete initiator system includes a redox system, i.e., reducing agents and oxidizing agents in a proportion that yields polymerization initiating species. All of these initiator systems are well known in the art.
Emulsion polymerizations are normally accomplished in the range of 5xc2x0 C. to 90xc2x0 C. Though the activator or xe2x80x9credoxxe2x80x9d initiated systems are preferred for low temperature polymerizations, they are very effective at high temperatures also, normally requiring appreciably lower quantities of the various ingredients to obtain a desirable polymerization rate.
The free radical sources used in the initiator systems are those customarily used in free radical polymerizations, for example, organic initiators such as azonitriles, azo-derivatives, peroxides and hydroperoxides and inorganic initiators such as inorganic peroxy compounds. Radiation, e.g., of the ultraviolet and gamma ray type can also be used as a free radical source. Various organic initiators are described by J. Brandrup and E. H. Immergut, Polymer Handbook (John Wiley and Sons), 1965, pages II-3 to II-51. The pH of the latex emulsion generally ranges from about 7.5 to 13. Preferably, the pH ranges from about 8 to 11.
The SBR and NBR latices emulsion may also contain various conventional compounds such as surface active agents, short stop materials such as di-tertiary-butyl hydroquinone and similar compounds, all of which are present in small amounts. The surface active agents may consist of emulsifiers comprising the salts of natural acids such as potassium stearate, potassium-processed rosin, and the like, or such surface active agents may be synthetics such as alkali metal salts of alkyl aryl sulfonic acid, particularly alkylbenzene sulfonic acid, condensed naphthalene-formaldehyde sulfonic acid, and the like, or non-ionics such as polyalkylene oxide dioleates, sorbitan trioleate, alkylolamides, the condensation products of nonyl phenol with ethylene oxide or propylene oxide-ethylene oxide products. Preferably, anionic-type surfactants are used such as potassium stearate, potassium-processed rosin, and the alkali metal salts of alkylbenzene sulfonic acid.
The SBR latex and the NBR latex may be blended in a wide range of weight ratios, based on dry weight. For example, the blend of the two latices may comprise from 99 to 30 percent by weight of the styrene butadiene rubber and from 1 percent to 70 percent by weight of the NBR rubber. Preferably, the ratio will range from 95 to 35 percent by weight of the styrene butadiene rubber and from 5 to 65 percent by weight of the NBR rubber.
In addition to the SBR latex and NBR latex, additional latices may be optionally added to form the blend of latices prior to coagulation and drying. For example, natural rubber latex, polybutadiene latex and mixtures thereof may be used. Relative to the total dry weight of the SBR latex and NBR, these optional latices are added in minor amounts. For example, based on a dried weight basis, the amount of optional latex, if used, may be combined in an amount ranging from about 5 to 45 weight percent of the total weight of the SBR latex, NER latex and optional combined. Preferably, if an optional latex or latices are used, the amount ranges from 10 to 30 percent by weight based on the total weight of the SBR, NBR and optional latices combined. The solids content of the natural rubber latex and polybutadiene latex may range from about 10 to 60. Preferably, the solids content ranges from 15 to 50.
After the NBR and SBR latices have been combined and thoroughly mixed, along with any optional latex or latices, the blend of latices are coagulated to recover a dry rubber. Standard coagulation techniques, such as salt-acid coagulation procedures, can be employed.
The minority (from 0 phr to 50 phr) of the total rubber used in the tread is dried rubber(s) not derived from a blend of two or more latices. This rubber(s) may be natural rubber, synthetic cis 1,4-polyisoprene, 3,4-polyisoprene, solution polymerized or emulsion polymerized styrene/butadiene rubber, styrene/isoprene/butadiene terpolymer rubber, cis 1,4-polybutadiene, high trans 1,4-polybutadiene, styrene/isoprene rubber, isoprene/butadiene rubber and mixtures thereof. Preferably, if used, the dried rubber is natural rubber, cis-1,4-polybutadiene and mixtures thereof. The preferred amount of this component rubber is from 10 to 40 phr of the total rubber used in the tread with the balance of 90 to 60 phr being derived from the second blend of latices.
The term xe2x80x9cphrxe2x80x9d as used herein, and according to conventional practice, refers to xe2x80x9cparts by weight of a respective material per 100 parts by weight of rubber or elastomer.xe2x80x9d
The pneumatic tire of the present invention may contain a siliceous filler. The siliceous fillers or pigments include pyrogenic and precipitated silica. The siliceous pigments are preferably precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate.
The silica filler, if used, may be added in amounts ranging from 10 to 250 phr. Preferably, the silica is present in an amount ranging from 15 to 80 phr.
Such silicas might be characterized, for example, by having a BET surface area, as measured using nitrogen gas, preferably in the range of about 40 to about 600, and more usually in a range of about 50 to about 300 square meters per gram. The BET method of measuring surface area is described in the Journal of the American Chemical Society, Volume 60, page 304 (1930).
The silica may also be typically characterized by having a dibutylphthalate (DBP) absorption value in a range of about 100 to about 400, and more usually about 150 to about 300.
The silica might be expected to have an average ultimate particle size, for example, in the range of 0.01 to 0.05 micron as determined by the electron microscope, although the silica particles may be even smaller, or possibly larger, in size.
Various commercially available silicas may be considered for use in this invention such as, only for example herein, and without limitation, silicas commercially available from PPG Industries under the Hi-Sil trademark with designations 210, 243, etc; silicas available from Rhone-Poulenc, with, for example, designations of Z1165MP and Z165GR and silicas available from Degussa AG with, for example, designations VN2 and VN3, etc. The PPG Hi-Sil silicas are currently preferred.
If silica is optionally added to the compound, one may add silica coupling agents to promote the adhesion between the rubber and silica. Representative coupling agents include organosilanes such as 3,3xe2x80x2 bis(triethoxysilylpropyl) tetrasulfide. Silica coupling agents are generally used in amounts ranging from 0.1 to 20 phr. Specific examples of such organosilanes are disclosed in U.S. Pat. No. 4,128,438 incorporated by reference in its entirety.
It is readily understood by those having skill in the art that the rubber composition would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents and reinforcing materials such as, for example, carbon black. As known to those skilled in the art, depending on the particular properties of the tire tread, the additives mentioned above are selected and commonly used in conventional amounts.
Typical amounts of reinforcing type carbon blacks(s), range from 0 to 200 phr. Preferably, the carbon black level ranges from 5 to 125 phr. Representative of the conventional carbon blacks which may be used in the present invention include those known to those skilled in the art under the ASTM designations N110, N121, N205, N220, N231, N234, N242, N293, N299, S315, N326, N330, M332, N339, N343, N347, N351, N358 and N375. Typical amounts of tackifier resins, if used, comprise about 0.5 to about 10 phr, usually about 1 to about 5 phr. Typical amounts of processing aids comprise about 1 to about 50 phr. Such processing aids can include, for example, aromatic, napthenic, and/or paraffinic processing oils. Typical amounts of antioxidants comprise about 1 to about 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others, such as, for example, those disclosed in the Vanderbilt Rubber Handbook (1978), pages 344-346. Typical amounts of antiozonants comprise about 1 to 5 phr. Typical amounts of fatty acids, if used, which can include stearic acid comprise about 0.5 to about 3 phr. Typical amounts of zinc oxide comprise about 2 to about 5 phr. Typical amounts of waxes comprise about 1 to about 5 phr. Often microcrystalline waxes are used. Typical amounts of peptizers comprise about 0.1 to about 1 phr. Typical peptizers may be, for example, pentachlorothiophenol and dibenzamidodiphenyl disulfide.
The vulcanization is conducted in the presence of a sulfur vulcanizing agent. Examples of suitable sulfur vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide or sulfur olefin adducts. Preferably, the sulfur vulcanizing agent is elemental sulfur. As known to those skilled in the art, sulfur vulcanizing agents are used in an amount ranging from about 0.5 to about 4 phr, or even, in some circumstances, up to about 8 phr, with a range of from about 1.5 to about 2.5, sometimes from 2 to 2.5, being preferred.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. Conventionally and preferably, a primary accelerator(s) is used in total amounts ranging from about 0.5 to about 4, preferably about 0.8 to about 1.5, phr. In another embodiment, combinations of a primary and a secondary accelerator might be used with the secondary accelerator being used in smaller amounts (of about 0.05 to about 3 phr) in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators might be expected to produce a synergistic effect on the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce a satisfactory cure at ordinary vulcanization temperatures. Vulcanization retarders might also be used. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
Vulcanization of the rubber composition of the present invention is generally carried out at conventional temperatures ranging from about 100xc2x0 C. to 200xc2x0 C. Preferably, the vulcanization is conducted at temperatures ranging from about 110xc2x0 C. to 180xc2x0 C. Any of the usual vulcanization processes may be used such as heating in a press or mold, heating with superheated steam or hot air or in a salt bath.
The mixing of the rubber composition can be accomplished by methods known to those having skill in the rubber mixing art. For example the ingredients are typically mixed in at least two stages, namely at least one non-productive stage followed by a productive mix stage. The final curatives are typically mixed in the final stage which is conventionally called the xe2x80x9cproductivexe2x80x9d mix stage in which the mixing typically occurs at a temperature, or ultimate temperature, lower than the mix temperature(s) than the preceding non-productive mix stage(s). The rubber, silica and silica coupler, and carbon black if used, are mixed in one or more non-productive mix stages. The terms xe2x80x9cnon-productivexe2x80x9d and xe2x80x9cproductivexe2x80x9d mix stages are well known to those having skill in the rubber mixing art.
When large amounts of silica (over 20 phr) are used in conjunction with coupling agents, extended mix times significantly improve physical properties. Preferably, the mechanical mixing is under conditions sufficient to generate a rubber temperature between 130xc2x0 C. and 180xc2x0 C. for a period of time ranging from 10 seconds to 20 minutes.
The rubber compound as described herein is particularly suited for a tread of a pneumatic tire. Such tires can be built, shaped, molded and cured by various methods which are known and will be readily apparent to those having skill in such art. As can be appreciated, the tire may be a passenger tire, aircraft tire, truck tire and the like. Preferably, the tire is a passenger tire. The tire may also be a radial or bias, with a radial tire being preferred.
The invention may be better understood by reference to the following examples in which the parts and percentages are by weight unless otherwise indicated.